DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides an improved method for drying of SAP so as to produce super absorbent polymers (SAP) with low residual monomer (RM) content.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] A superabsorbent polymer is a cross-linked partially neutralized polymer, including cross-linked polyacrylic acids, capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer. Superabsorbent polymer may be formed into particles, generally referred to as particulate superabsorbent polymer. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, and particles hereof. A primary use of superabsorbent polymer and superabsorbent polymer compositions is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
[0004] Typically, the superabsorbent polymers (SAPs) are made at an industrial scale by reacting ethylenically unsaturated carboxylic acid monomer or its sodium salt and combination of cross-linker of different chain lengths to produce the superabsorbent polymers (SAPs). For polymerization, radical initiators such as persulfate salt, hydrogen peroxide and organic peroxide are generally used. The resultant polymer gel is micronized and dried by using one or more drying equipments such as belt dryer, drum dryer, fluidized bed dryer etc. The resultant dried polymer is then crushed to the particle size ranging from 150 to 850 micron to get the required water absorption capacity. The resultant SAP particles are then tested in terms of its Absorption Under Load (AUL), Centrifuge Retention Capacity (CRC) and other properties for further finishing stages.
[0005] In order to get good absorption properties for SAP, typically acrylic acid of more than 99.95% purity is used. But due to storage conditions (especially in temperate regions), the acrylic acid dimer present in the acrylic acid increases. Usage of acrylic acid containing dimer impurity leads to inferior product with low absorbency, higher extractables and increased residual monomer (RM) content due to dimer breakage, and the likes. Particularly, high RM content in the SAP is disadvantageous for its utility in the end applications, such as in production of sanitary articles, like babies' diapers, incontinence products, or sanitary towels.
[0006] Accordingly, there remained a long-felt need in the art to device an improved method of drying of SAP so as to effect production of SAP with low residual monomer content.

OBJECTS OF THE INVENTION
[0007] An object of the present disclosure is to provide a method of drying of SAP that affords production of SAP with low residual monomer content.
[0008] Another object of the present disclosure is to provide a method of production of polyacrylate SAP that affords production of SAP with low residual monomer content.
[0009] Further object of the present disclosure is to provide a method of production of polyacrylate SAP that is economical.
[0010] Still further object of the present disclosure is to provide a process that is technically and commercially feasible.
[0011] Other objects of the present invention will be apparent from the description of the invention herein below.

SUMMARY
[0012] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides an improved method for drying of SAP so as to produce super absorbent polymers (SAP) with low residual monomer (RM) content.
[0013] An aspect of the present disclosure relates to a method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 500 ppm based on dry weight of the SAP, the method comprising the steps of: taking acrylic acid monomer; contacting the acrylic acid monomer with one or a combination of cross-linking agents; effecting addition of a redox initiator, and a thermal polymerization initiator to produce polyacrylate superabsorbent polymer (SAP); and drying the polyacrylate superabsorbent polymer to produce the polyacrylate SAP with residual monomer content of less than 500 ppm, the step of drying comprising: (a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature ranging from 175°C to 185°C at a first air flow rate ranging from 1.5 m/s to 2.5 m/s for a first time period; and (b) exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature ranging from 150°C to 170°C at a second air flow rate ranging from 1.5 m/s to 2.5 m/s for a second time period.
[0014] In an embodiment, the acrylic acid monomer comprises any of: acrylic acid, glacial acrylic acid and a salt of acrylic acid. In an embodiment, the acrylic acid monomer comprises glacial acrylic acid, and wherein the glacial acrylic acid is neutralized before contacting the glacial acrylic acid with the one or a combination of cross-linking agents, further wherein the glacial acrylic acid is neutralized from 50 mol% to 85 mol%. In an embodiment, the one or a combination of cross-linking agents are selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent. In an embodiment, the redox initiator comprises: (a) a reducing agent selected from ascorbic acid, sodium ascorbate, sulfite of alkali metal, bisulfite of alkali metal, a sugar, an aldehyde, a primary alcohol and a secondary alcohol; and (b) an oxidizing agent selected from hydrogen peroxide, an alkyl peroxide selected from t-butyl hydroperoxide, benzoyl peroxide, and 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; and sodium peracetate. In an embodiment, the thermal polymerization initiator comprises any of a combination of: inorganic peroxides and t-alkyl peroxides. In an embodiment, the thermal polymerization initiator comprises one or more inorganic peroxides. In an embodiment, the first time period ranges from 5 minutes to 30 minutes, and the second time period ranges from 10 minutes to 40 minutes. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 500 ppm to 10000 ppm. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 300 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles. In an embodiment, the drying was effected in a hot air convection type dryer. In an embodiment, the hot air is passed in the upward direction during the first time period and the hot air is passed in the downward direction during the second time period. In an embodiment, depth of the polyacrylate superabsorbent polymer (SAP) ranges from 70 mm to 120 mm. In an embodiment, open area of drying tray ranges from 20% to 30%.

BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 illustrates a graph showing RM content of SAP produced using acrylic acid feed with varying dimer content.

DETAILED DESCRIPTION
[0016] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.
[0017] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the “invention” may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0018] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0019] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0020] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0021] The term “SAP” or “superabsorbent polymer” as used herein synonymously and interchangeably, throughout the present disclosure, denotes natural, semi-synthetic or synthetic polymeric materials that can absorb large amount of a liquid relative to its own mass. Such superabsorbent polymer (SAP) may be water-swellable, water-insoluble organic or inorganic materials including superabsorbent polymers and superabsorbent polymer compositions capable, under the most favorable conditions, of absorbing at least about 10 times their weight, or at least about 15 times their weight, or at least about 25 times their weight in an aqueous solution containing 0.9 weight percent sodium chloride.
[0022] The term “acrylic acid” or “acrylic acid monomers” or “monomer composition” as used herein synonymously and interchangeably throughout the present disclosure denotes the acrylic acid monomers, glacial acrylic acid monomers, salt of acrylic acid monomers such as monovalent metal salts, divalent metal salts, ammonium salts or organic amine salts of acrylic acid and the like monomers with acrylate moiety that may find utility in preparation of the polymer, particularly, the polyacrylate polymers.
[0023] The term “residual monomer content” or “RM content” as used herein synonymously and interchangeably throughout the present disclosure denotes the amount of monomers that are remained in the SAP. Specifically, it is an amount (in ppm) obtained by adding 1.000 g of SAP to 200 mL of 0.9% by weight sodium chloride aqueous solution, stirring for one hour, and then measuring the amount of residual monomer dissolved into the solution by high performance liquid chromatography (HPLC).
[0024] The term “Absorption Under Load” or “AUL” as used herein synonymously and interchangeably, throughout the present disclosure, is a measure of the ability of a superabsorbent polymer to absorb fluid under an applied pressure and is stated as grams of liquid absorbed per gram weight of the sample (g/g) at an applied pressure. AUL is determined as follows: 0.9 g of weighed SAP sample is placed in a plastic cylinder having inner diameter of 6 cm and height of 5 cm with a nylon screen fabric (mesh size 400 mesh) at bottom. The SAP particles are uniformly dispersed and initial weight of the setup was measured (A). A Teflon made plastic plate with a metal piston is placed on test substance. Thereafter, the entire testing setup is placed on a ceramic filter plate (porosity=0) covered with a Whatman filter paper (porosity =25) and soaked with 0.9% NaCl solution upto top edge of filter plate and the SAP sample is allowed to absorb liquid for 60 minutes. Afterwards, the testing set up is slowly moved out of NaCl solution and gently remove the wet SAP sample which is stuck to the Teflon plate. The weight of the swollen testing setup without Teflon plate and metal piston was recorded (B). The gram amount of the NaCl solution that had been retained per gram of sample was calculated according to the following equation:
AUL=
Where AUL is in g/g at 0.3 psi and C is the actual weight of SAP sample in grams.
[0025] The term “Centrifuge Retention Capacity” or “CRC” as used herein synonymously and interchangeably, throughout the present disclosure, is the ability of the particulate superabsorbent polymer (SAP) to retain liquid therein after being saturated and subjected to centrifugation under controlled conditions and is stated as grams of liquid retained per gram weight of the sample (g/g). CRC testing may be conducted at an assigned testing temperature for an assigned period of testing time, noted as CRC(testing temperature, testing time). For example, CRC(23° C, 0.5 h) refers to a CRC with a testing temperature of about 23°Cand a testing time of 0.5 hour. CRC of SAP samples is determined by standard method no. ISO 17190-6. 0.2 g of weighed SAP samples are placed in non-woven bags of dimension 6 cm X 8 cm and submerged in beaker containing 0.9% NaCl solution for half an hour. Empty bags are used as controls and treated in similar way. The bags are then placed in centrifuge basket and centrifuged at 250 g for 3 min. The bags are removed and weighed. CRC of the samples are measured by using following equation:
CRC = (w1 – w2 –w3) / w3
Where, w1 is the weight of wet sample and bag, w2 is the weight of empty bag and w3 is the weight of dry sample.
[0026] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides an improved method for drying of SAP so as to produce super absorbent polymers (SAP) with low residual monomer (RM) content.
[0027] In order to mitigate the problem of high RM content of SAP, various changes in the feedstock composition (change in ingredients and concentration thereof for production of SAP) was attempted to enhance controlled radical polymerization by increasing the cross linker content, but the same was found to be disadvantageous owing to deterioration of absorption properties such as AUL and CRC of the resultant polymer (SAP). Further, it is observed that increase in cross-linking negatively affects drying. It could also be observed that chain transfer property of the dimer leads to lower molecular mass of the polymer, which contributed to extractables. Hence, inventors of the present disclosure resorted to develop an improved drying method, which does not affect properties of the polymer (SAP).
[0028] In the present disclosure, a physical process is employed by which the residual monomer in the base polymer (SAP) is removed by optimizing the drying conditions. Particularly, inventors of the present application surprisingly observed that with proper control over humidity and temperature during drying, RM content of the SAP can be reduced to an acceptable level. It could also be surprisingly observed that cracking of dimer residues to acrylic acid monomer at the drying temperatures also contributes to the RM content. During experiments, it could be surprisingly observed that maintaining higher drying temperature in the initial period of drying aids in the dimer breakage (i.e. conversion of dimer to monomers) and removal thereof when the polymer (SAP) is still in the rubbery state, while towards later part of drying, rate of residual monomer (RM) removal is low.
[0029] Accordingly, an aspect of the present disclosure relates to a method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 500 ppm based on dry weight of the SAP, the method comprising the steps of: taking acrylic acid monomer; contacting the acrylic acid monomer with one or a combination of cross-linking agents; effecting addition of a redox initiator, and a thermal polymerization initiator to produce polyacrylate superabsorbent polymer (SAP); and drying the polyacrylate superabsorbent polymer to produce the polyacrylate SAP with residual monomer content of less than 500 ppm, the step of drying comprising: (a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature ranging from 175°C to 185°C at a first air flow rate ranging from 1.5 m/s to 2.5 m/s for a first time period; and (b) exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature ranging from 150°C to 170°C at a second air flow rate ranging from 1.5 m/s to 2.5 m/s for a second time period.
[0030] In an embodiment, the drying was effected in a hot air convection type dryer. In an embodiment, the hot air is passed in the upward direction during the first time period and the hot air is passed in the downward direction during the second time period. In an embodiment, depth of the polyacrylate superabsorbent polymer (SAP) bed ranges from 70 mm to 120 mm. In an embodiment, open area of drying tray ranges from 20% to 30%.
[0031] In an embodiment, the acrylic acid monomer comprises any of: acrylic acid, glacial acrylic acid and a salt of acrylic acid. In an embodiment, the acrylic acid monomer comprises glacial acrylic acid, and wherein the glacial acrylic acid is neutralized before contacting the glacial acrylic acid with the one or a combination of cross-linking agents, further wherein the glacial acrylic acid is neutralized from 50 mol% to 85 mol%. In an embodiment, the one or a combination of cross-linking agents are selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent. In an embodiment, the redox initiator comprises: (a) a reducing agent selected from ascorbic acid, sodium ascorbate, sulfite of alkali metal, bisulfite of alkali metal, a sugar, an aldehyde, a primary alcohol and a secondary alcohol; and (b) an oxidizing agent selected from hydrogen peroxide, an alkyl peroxide selected from t-butyl hydroperoxide, benzoyl peroxide, and 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; and sodium peracetate. In an embodiment, the thermal polymerization initiator comprises any of a combination of: inorganic peroxides and t-alkyl peroxides. In an embodiment, the first time period ranges from 5 minutes to 60 minutes, and the second time period ranges from 5 minutes to 60 minutes. In an embodiment, the first time period ranges from 5 minutes to 30 minutes, and the second time period ranges from 10 minutes to 40 minutes. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 500 ppm to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 500 ppm to 10000 ppm. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 400 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 300 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles. In an embodiment, the polyacrylate SAP particles are treated with a surface crosslinker, said surface crosslinker selected from cyclic carbonate esters, polyols, diglycidyl ethers and combination thereof.
[0032] In an embodiment, the method of production of SAP includes the steps of: taking acrylic acid monomer; contacting acrylic acid with one or a combination of cross-linking agents; effecting addition of a redox initiator and thermal polymerization initiator to produce polyacrylate superabsorbent polymer; and drying the polyacrylate superabsorbent polymer.
[0033] In an embodiment, the acrylic acid monomer comprises any of acrylic acid, glacial acrylic acid, and salt of acrylic acid. In an embodiment, the acrylic acid monomer comprises glacial acrylic acid, wherein the glacial acrylic acid is neutralized before contacting with the one or a combination of cross-linking agents. In an embodiment, the one or a combination of cross-linking agents is selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent.
[0034] In an embodiment, the redox initiator includes (a) a reducing agent, such as ascorbic acid, sodium ascorbate, sulfite or bisulfite of alkali metal, a sugar, an aldehyde or a primary or secondary alcohol, and (b) an oxidizing agent, such as hydrogen peroxide, an alkyl peroxide, like t-butyl hydroperoxide, benzoyl peroxide, 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; sodium peracetate; and other redox initiators known to persons skilled in the art.
[0035] In an embodiment, thermal polymerization initiator is selected from any or a combination of inorganic peroxides and t-alkyl peroxides. In an embodiment, thermal polymerization initiator includes one or more inorganic peroxides.
[0036] In an embodiment, the acrylic acid monomer includes dimer content ranging from 500 ppm to 30000 ppm. In an embodiment, the acrylic acid monomer includes dimer content ranging from 500 ppm to 10000 ppm. In an embodiment, the acrylic acid monomer includes dimer content ranging from 3000 ppm to 10000 ppm. In an embodiment, the acrylic acid monomer includes dimer content ranging from 3000 ppm to 7000 ppm. In an embodiment, the acrylic acid monomer includes dimer content ranging from 5000 ppm to 10000 ppm. In an embodiment, the acrylic acid monomer includes dimer content ranging from 7000 ppm to 10000 ppm.
[0037] In an embodiment, the SAP polymerization is effected in a continuous kneader polymerization reactor. Such reactors are known to a person skilled in the art, for example, kneader reactors as disclosed in CH-A 664704, EP-A 517068, WO 97/12666, DE-A 2123956, EP-A 603525, DE-A 19536944 and DE-A 18884 may be suitable.
[0038] In an embodiment, the moisture content of the resultant polymer gel (SAP) ranges from 55% to 70%, such that the polymer gel does not clump while transferring from polymerization reactor into dryer.
[0039] In an embodiment, drying of the superabsorbent polymer is effected by exposing the superabsorbent polymer (SAP) to a first temperature ranging from about 170°C to about 200°C and at a first air velocity ranging from about 1.5 m/s to about 2.5 m/s for a first time period, followed by exposing said polyacrylate superabsorbent polymer (SAP) to a second temperature ranging from about 150°C to about 170°C at a second air flow velocity for a second time period. In an embodiment, the method affords production of SAP with residual monomer content of less than about 500 ppm. In an embodiment, the method affords production of SAP with residual monomer content of less than about 400 ppm. In an embodiment, the method affords production of SAP with residual monomer content of less than about 300 ppm.
[0040] In an embodiment, the first temperature ranges from about 175°C to about 185°C. In an embodiment, the first temperature is about 185°C. In an embodiment, the second temperature ranges from about 160°C to about 170°C. In an embodiment, the second temperature is about 165°C. In an embodiment, the first air flow velocity ranges from about 1.8 m/s to about 2.2 m/s. In an embodiment, the first air flow velocity is about 2.0 m/s. In an embodiment, the second air flow velocity ranges from about 1.5 m/s to about 2.5 m/s. In an embodiment, the second air flow velocity ranges from about 1.7 m/s to about 2.0 m/s. In an embodiment, the second air flow velocity is about 1.9 m/s. In an embodiment, the first time period ranges from about 5 minutes to about 60 minutes. In an embodiment, the first time period ranges from about 5 minutes to about 30 minutes. In an embodiment, the second time period ranges from about 5 minutes to about 60 minutes. In an embodiment, the second time period ranges from about 10 minutes to about 40 minutes. In an embodiment, the first time period is about 10 minutes. In an embodiment, the second time period is about 15 minutes. In an embodiment, the superabsorbent polymer (SAP) has residual monomer content of less than 300 ppm based on dry weight of the SAP.
[0041] In an embodiment, hot air convection type dryer with means to control or adjust the flow rate of hot air, as known to or appreciated by a person skilled in the art, is used for the purpose of effecting drying of the polymer. In an embodiment, hot air is passed in the upward direction during the first time period (i.e. during the initial drying stage).In an embodiment, hot air is passed in the downward direction during the second time period (i.e. during the later drying stage). Change in direction of passage of hot air aids in uniform drying of the polymer. Further, during the initial stage of drying, the drying temperature may be higher than that during the later stage of drying. Lower temperature during the later stage of drying (second time period) may be advantageous, as otherwise, continued exposure of SAP to higher temperature may result in deterioration of properties thereof. In an embodiment, moisture content of the dried SAP/polymer is less than about 10%, preferably less than about 5% and more preferably, less than about 3%.
[0042] In an embodiment, the polymer gel bed depth/height is kept in the range of 70 mm to 120 mm for effective removal of residual monomers (that may be present in the gel or the one formed by disintegration of the dimer). In an embodiment, the polymer gel (SAP) bed depth/height is kept in the range of 80 mm to 100 mm. In an embodiment, the polymer gel (SAP) bed depth/height is kept in the range of 70 mm to 80 mm. In an embodiment, the polymer gel (SAP) bed depth/height is kept in the range of 100 mm to 120 mm. The polymer gel (SAP) depth depends on the amount of crosslinking. If the polymer (SAP) is heavily cross-linked (> 12800 ppm) bed depth of 70-80 mm may be optimum for drying. At lower cross-linking levels (>9500 and <11000), bed depths of 80 mm to 100 mm may be optimum for drying. In an embodiment, the open area of drying tray ranges from 20% to 30%. In an embodiment, the open area of drying tray ranges from 23% to 26% for effective removal of residual monomers.
[0043] In an embodiment, the dried superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles. In an embodiment, the SAP particles are further treated with a surface crosslinker. In an embodiment, the surface crosslinker is selected from bi-functional molecules such as ethylene carbonate, diols or combination thereof.
[0044] Experiments were performed with polymerization of acrylic acid (monomer feedstock) with dimer content varying from 500 ppm up to 15000 ppm. It was found that dimer worked as chain transfer agent, reducing the molecular weight. The dimer gets converted to monomer during the thermal treatment in the drying step and subsequent surface cross-linking step (as it includes subjecting SAP to thermal treatment).
[0045] SAP POLYMERIZATION
[0046] The SAP polymerization involved usage of crosslinker Trimethylol propane triacrylate (ethoxylated) at a concentration of 4000-15000 ppm, Potassium persulfate initiator at a concentration of 1000-5000 ppm, Sodium Ascorbate initiator at a concentration of 50-400 ppm and hydrogen peroxide initiator at a concentration of 300-1000 ppm. Feed was prepared by neutralizing acrylic acid with sodium hydroxide and mixing crosslinker in it. Continuous kneader polymerization reactor (commercially available from M/s. LIST TECHNOLOGY AG.) was used for effecting polymerization. With optimum crosslinking and initiator concentration as described above, the fill level in the kneader polymerization reactor was measured in the region of solid gel. The fill level of about 80% was used, so as to reduce agglomeration of the gel particles and to keep tight control over crumb size and quality of the polymer gel discharged from the polymerization reactor. The fill height was controlled by the variation of discharge weir (discharge spout/ barrier) at the kneader outlet. The moisture content of the free-flowing polymer gel (product) was maintained between 55% and 70% to preclude clumping while transferring from polymerization reactor into the dryer.The residual monomer content of the polymer gel was measured to quantify residual monomer removal at each step of the process.
[0047] DRYING OF POLYMER GEL
[0048] Hot air convection type dryer (Batch tray dryer from Theilen Machinenbau, Germany) was used for effecting drying of the resultant polymer gel. Overall drying time was about 25 minutes, divided into two phases/stages, namely initial phase/stage and later phase/stage.
[0049] Optimization of Drying conditions
[0050] Different experiments were performed to understand effect of different drying conditions, results whereof are shown in Table 1 below. In these experiments, the time period for the 1st phase was about 10 minutes and the time period for the 2nd phase was about 15 minutes (i.e. overall drying period was about 25 minutes divided in 2 phases). The drying was effected in a dry air convection type dryer, the bed depth (height) was maintained at about 80 mm, open area of the drying tray was kept at about 25% (with tray drying area of about 0.1 m2) and air flow rate was maintained at 1.9 m/s (upward or downward as per the experimental protocol).
Table 1: Effect of different drying conditions on properties of the SAP
S. No. Drying conditions
(1st Phase; 2nd Phase) AUL(0.3)
(g/g) CRC
(g/g) RM
(ppm) Extractable
(%)
1
160 oC up flow;
160 oC down flow 19 34 184 6.8
2 180 oC up flow;
180 oC down flow 12 33 186 8.7
3 200 oC up flow;
200 oC down flow 15 34 250 9.2
4 170 oC up flow;
150 oC down flow 17 34 220 6.1
5 180 oC up flow;
160 oC down flow 16 35 212 5.4
6 160 oC up flow;
180 oC down flow 17 35 637 6.3

[0051] It could be surprisingly noted that (i) even if the drying is done at a constant lower temperature, it does not solve the problem (see entry 1 in the Table 1 above); (ii) further, if the drying is done at a constant high temperature (see entry 3 in the Table 1 above), it increases the extractables defeating the purpose. Hence, drying needs to be done in phases. It could also be observed, albeit surprisingly, that in the 1stphase, drying needs to be done at higher temperature and in the 2ndphase, drying needs to be done at a lower temperature. This aids in preventing disintegration of lower molecular weight portion of the polymer.
[0052] Drying Temperature and Flow rate of hot air
[0053] Drying of the polymer was carried out at a temperature of about 185oC with flow velocity of the hot air maintained at about 2 m/s in the initial stage for 10 minutes. During initial stage (1st phase) of drying, hot air was passed in the upward direction, and during the later stage (2nd phase) of drying (15 minutes), hot air was passed in downward direction to achieve uniform drying with the total drying time of about 25 minutes. Temperature of the bed during the later stage of drying was set to 165oC (with air flow rate of about 1.9 m/s) in order to preclude disintegration of lower molecular weight portion of the polymer. It could be noted that continuous exposure of polymer to high temperature increases the extractable (low molecular weight polymers).
[0054] For samples obtained with acrylic acid feed with different dimer content, it was observed that under optimized drying conditions (80 mm bed length, 1.8 m/s air flow, 185 upward flow for 10 min followed by 165 downward flow for 15 min) acrylic acid with dimer content up to 1% can be converted into the final SAP product with less than 300 ppm residual monomer. It could also be observed that if the initial dimer content is higher than 1%, RM decreases and then marginally increases. This may be due to the decomposition of the dimer trapped in the final SAP samples, which may not be removed. FIG. 1 illustrates a graph showing RM content of SAP produced using acrylic acid feed with varying dimer content.
[0055] Polymer Bed Height and Open Area
[0056] The polymer gel bed depth (height) was maintained at about 80 mm for effective removal of residual monomers present in the gel (SAP) as well as the monomer formed by disintegration of dimer. It could be observed that when the bed height was less than 70 mm, channeling of the hot air occurs in the tray, which resulted in SAP with higher RM content. When the polymer gel bed height was more than 120 mm, drying efficiency was reduced and dimer and/or residual monomer removal was not efficient. The polymer gel depth/height was also found to be dependent on the amount of crosslinking of the polymer. If the polymer is heavily cross-linked (i.e. cross-linking > 12800 ppm) bed height/depth of 75-80 mm was found to be optimum, and for polymers with lower cross-linking levels (more than 9720 ppm but less than 10800 ppm), bed depth/height of 80-100 mm was found to be optimum for drying and effective removal of residual monomer (RM).
[0057] The open area of the drying tray was kept at about 25% (with tray drying area of about 0.1 m2) for effective removal of residual monomers. It was found during the experiments that with higher % of open area of the tray, residual monomer removal was not adequate.
[0058] The final moisture content of the SAP was found to be less than 5%. Dried SAP was then analyzed for residual monomer (RM) content.
[0059] Accordingly, the method of the present disclosure affords reduction of the residual monomer (RM) levels from above 10000 ppm to below 300 ppm.

ADVANTAGES
[0060] The present disclosure provides a method of drying of SAP that affords production of SAP with low residual monomer content.
[0061] The present disclosure provides a method of production of polyacrylate SAP that that affords production of SAP with low residual monomer content.
[0062] The present disclosure provides a method of production of polyacrylate SAP that is economical.
[0063] The present disclosure provides a process that is technically and commercially feasible.

,CLAIMS:1. A method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 500 ppm based on dry weight of the SAP, the method comprising the steps of:
taking acrylic acid monomer;
contacting the acrylic acid monomer with one or a combination of cross-linking agents;
effecting addition of a redox initiator, and a thermal polymerization initiator to produce polyacrylate superabsorbent polymer (SAP); and
drying the polyacrylate superabsorbent polymer to produce the polyacrylate SAP with residual monomer content of less than 500 ppm, the step of drying comprising:
(a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature ranging from 175°C to 185°C at a first air flow rate ranging from 1.5 m/s to 2.5 m/s for a first time period; and
(b) exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature ranging from 150°C to 170°C at a second air flow rateranging from 1.5 m/s to 2.5 m/s for a second time period.
2. The method as claimed in claim 1, wherein the acrylic acid monomer comprises any of: acrylic acid, glacial acrylic acid and a salt of acrylic acid.
3. The method as claimed in claim 1, wherein the acrylic acid monomer comprises glacial acrylic acid, and wherein the glacial acrylic acid is neutralized before contacting the glacial acrylic acid with the one or a combination of cross-linking agents, further wherein the glacial acrylic acid is neutralized from 50 mol% to 85 mol%.
4. The method as claimed in claim 1, wherein the one or a combination of cross-linking agents are selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent.
5. The method as claimed in claim 1, wherein the redox initiator comprises:
(a) a reducing agent selected from ascorbic acid, sodium ascorbate, sulfite of alkali metal, bisulfite of alkali metal, a sugar, an aldehyde, a primary alcohol and a secondary alcohol; and
(b) an oxidizing agent selected from hydrogen peroxide, an alkyl peroxide selected from t-butyl hydroperoxide, benzoyl peroxide, and 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; and sodium peracetate.
6. The method as claimed in claim 1, wherein the thermal polymerization initiator comprises one or more inorganic peroxides.
7. The method as claimed in claim 1, wherein the first time period ranges from 5 minutes to 30 minutes, and the second time period ranges from 10 minutes to 40 minutes.
8. The method as claimed in claim 1, wherein the acrylic acid monomer comprises dimer content ranging from 500 ppm to 10000 ppm.
9. The method as claimed in claim 1, wherein the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 300 ppm based on dry weight of the SAP.
10. The method as claimed in claim 1, wherein the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles.
11. The method as claimed in claim 1, wherein the drying was effected in a hot air convection type dryer.
12. The method as claimed in claim 11, wherein the hot air is passed in the upward direction during the first time period and the hot air is passed in the downward direction during the second time period.
13. The method as claimed in claim 11, wherein depth of the polyacrylate superabsorbent polymer (SAP) ranges from 70 mm to 120 mm.
14. The method as claimed in claim 11, wherein open area of drying tray ranges from 20% to 30%.